Heretofore, for connecting ICs or LSIs in an electric circuit substrate, a method has been used in which individual pins are soldered to a print wiring board. However, this method is of low production efficiency and is not suitable for densification.
In order to improve connection reliability, developed were technologies such as a BGA (ball grid array) in which substrates are connected with a so-called solder ball, which is a solder shaped into a spherical form. According to this technology, it was possible to produce an electric circuit while reconciling high productivity and high connection reliability by connecting a substrate, a chip and a solder ball mounted on the substrate while melting them at high temperature.
However, multilayering of substrates has advanced recently, which causes a problem that strain or expansion and contraction of a substrate itself is caused by change of the external environment and, as a result, application of such force to connections of substrates will cause breakage of wire. Moreover, the multilayering has made it difficult to maintain the distance between substrates; the fact that maintaining of that distance requires installation of spacers or the like and therefore much time and cost are needed has caused a problem.
As means for solving these problems, with respect to the relaxation of the force applied to circuits of substrates or the like, an attempt to reinforce substrate connections by application of a resin or the like has been made and this has exhibited a certain effect in improving a connection reliability. However, it has a problem that much labor is required and the cost increase is caused by addition of the application step. On the other hand, with respect to the maintenance of the distance between substrates, when balls in which copper is coated with solder are used, the copper, which does not melt like solder, serves as a support and makes it possible to maintain the distance between substrates. However, since copper is expensive and heavy, materials are awaited which are inexpensive and lightweight.